In an injection molding apparatus, a manifold receives a pressurized melt stream from a machine nozzle. The manifold distributes the melt stream to a plurality of nozzles and the melt is forced through the nozzles and into a plurality of mold cavities. The melt is then cooled in the mold cavities and the molded parts are released so that another cycle can begin.
The amount of melt transferred to each nozzle can vary due to effects such as shear induced flow imbalance in the manifold, for example. In order to compensate for such effects and ensure that a sufficient amount of melt is delivered to each mold cavity, the pressure applied to the melt stream by the machine nozzle must be very high. For applications such as injection molding of thin walled vessels and micro-molding, even higher nozzle pressures are required in order to produce quality molded products. As a result, the machine nozzle must be very large in order to generate sufficient pressure to properly distribute the melt to the mold cavities. In many cases, however, increasing the size of the machine nozzle is not a practical solution. Alternative solutions for increasing the pressure generated in each individual nozzle are therefore desirable.
Precise measurement of the volume of melt transferred in each shot for thin walled molded parts and micro-molded parts is also very important. This presents a unique challenge particularly when dealing with micro molded parts, which typically weigh a fraction of a gram. Several prior art devices have been developed to control the volume of melt that is injected into a mold cavity. These devices have typically been employed when injecting more than one material into a single mold cavity and tend to be complex and costly to manufacture.
U.S. Pat. No. 5,112,212 to Akselrud et al. discloses a shooting pot, which is used as a metering device, for use in a co-injection molding apparatus. The shooting pot is located remote from the hot runner nozzle and is used to control the volume of one of the two molten materials injected into the cavity. The shooting pot includes a piston that is axially movable within a cylinder to force molten material from the cylinder into a nozzle, which leads to a mold cavity. The cylinder includes an inlet that delivers melt from a melt source to a reservoir, which is located in a lower end of the piston. The piston is rotatable to move the reservoir out of communication with the inlet to seal it off so that when the piston is lowered, a known volume of melt is forced into the mold cavity.
U.S. Pat. No. 4,863,369 to Schad et al. discloses an injection molding apparatus that uses a shooting pot to deliver a precisely measured quantity of melt to a mold cavity. A valve is located in a conduit between a melt source and each nozzle. Once the shooting pot and nozzle are filled with melt, the valve is closed and the mold gate is opened. A piston of the shooting pot advances until it bottoms out in a cylinder to deliver a precise quantity of melt to a mold cavity.
A disadvantage of shooting pots that are remotely located from the nozzle and the mold cavity is that the known or measured volume of melt may vary from one molding cycle to the next. This occurs because there is a large volume of melt that is located between the shooting pot and the mold cavity i.e. the melt in the nozzle, the melt in the manifold channel and the melt in the shooting pot. This large volume of melt introduces several variables. Minor deviations in temperature or pressure, for example, may result in significant variations of the known volume. The sizable distance between the shooting pot and the mold cavity further causes the melt to have a long residence time outside of the nozzle between the injection of one article to the next. This results in molded parts that are not of the highest quality because the temperature of the melt coming from the shooting pot may be either under heated or over heated.
It is therefore an object of the present invention to provide a metering device for a nozzle of an injection molding apparatus, which obviates or mitigates at least one of the above disadvantages.